In recent years, for example, as an optical system for optical storage, developments are carried out on optical head devices for recording and reading (referred to as “recording/reading”, hereinafter) information onto and from an optical recording medium such as a CD, a DVD, and a magneto-optic disk, as well as a high-density optical recording medium such as a BD (referred to as an “optical disk”, hereinafter). In optical devices including optical head devices, wave plates are widely employed for changing the polarization state of entering light. Examples of these include: a ¼-wave plate for converting linearly-polarized entering light into circularly polarized light; and a ½-wave plate for converting linearly-polarized entering light into linearly polarized light whose direction of electric field is different from the direction of electric field of the entering light.
For example, an optical head device employs a polarization beam splitter for separating, by deflection, a forward optical path extending from a light source such as a semiconductor laser to an optical disk and a returning optical path extending from the point of reflection by the optical disk to a photodetector for detecting the light. Here, for the purpose that the polarization beam splitter should achieve a high efficiency of light utilization, a ¼-wave plate is arranged in the optical path between the polarization beam splitter and the optical disk such that the linearly polarized light in the forward-going path and in the return path should be perpendicular to each other. That is, in the forward-going path, first linearly polarized light goes straight and is transmitted through the polarization beam splitter so as to be brought into circularly polarized light by the ¼-wave plate. In contrast, the light of return path reflected by the optical disk is brought into circularly polarized light of reverse polarization and is transmitted through the ¼-wave plate again so as to be brought into second linearly polarized light perpendicular to the first linearly polarized light and then reflected by the polarization beam splitter. Thus, the light is guided to the photodetector with a high efficiency of light utilization.
The ¼-wave plate is arranged as a discrete optical element in the optical head device. However, for the purpose of size reduction of the optical head device, it is considered that the ¼-wave plate is integrated with another optical element. For example, when the function corresponding to a ¼-wave plate is imparted to the surface of a (raise-up) mirror for changing the direction of travel of the laser from the light source by 90°, that is, when an optical element is employed that deflects the direction of travel of entering light by 90° and that converts linearly polarized light into circularly polarized light and circularly polarized light into linearly polarized light, size reduction is realized in the optical head device.
As such a wave plate for reflecting and deflecting entering light and having the function of a ¼-wave plate, an optical head device has been reported in which a wave plate for generating a phase difference of 1/7 wavelength for light entering in the normal direction of the plane is arranged at an inclination of 45° relative to the optical axis so that the function of reflection of light and the function of a ¼-wave plate are achieved (Patent Document 1).
Further, an optical head device has been reported that employs a wave plate in which entering light is not limited to light of one wavelength, that is, light of two mutually different wavelengths like light having a wavelength of 780 nm for CD and linearly polarized light having a wavelength of 650 nm for DVD are reflected and deflected and in which the function of a ¼-wave plate is provided (Patent Document 2).